J Mar Sci Technol DOI 10.1007/s00773-016-0369-y

ORIGINAL ARTICLE

Performance analysis of polymer electrolyte membrane (PEM) fuel cell stack operated under marine environmental conditions B. Viswanath Sasank1 • N. Rajalakshmi1 • K. S. Dhathathreyan1

Received: 29 July 2015 / Accepted: 16 January 2016 Ó JASNAOE 2016

Abstract The marine environmental condition, especially NaCl, has been identified as one of the major sources of contamination on the performance of open cathode Proton Exchange Membrane Fuel Cells (PEMFC) system, when the power source is based on fuel cells for marine applications like submarines, navy ships etc., In the present paper, we have studied the performance of PEMFCs under the marine environment for a longer duration and also the recovery mechanism of the PEMFC power pack after contamination. It has been observed that the NaCl is a major contaminant for PEMFC, compared to NOx and SOx, which are major contaminants for fuel cells operating in the land regions. We have observed a performance loss of 60 % in PEMFC, when operated for 48 h, due to poisoning of PEMFC by NaCl vapours. The recovery of the stack is attempted by repeated water washing on the cathode side of the fuel cell, presuming that the salts get deposited only on the surface of the electrodes and the performance is easily recoverable. The recovery mechanisms are analysed by constant-current discharging operation and by modified experimental methods and are reported here. The performance vagaries in fuel cells due to sea water contamination is also analysed by linear fit and it is found that the rate of power increment after water wash is higher than the rate of power increment, around 11.5 W/10 h compared to normal environmental conditions, which is 4.1 W/10 h.

& N. Rajalakshmi [email protected] 1

Centre for Fuel Cell Technology, International Advanced Research centre for Powder Metallurgy and New Materials (ARCI), IIT Madras Research Park, 6, Kanagam road, Taramani, Chennai 600113, India

Keywords Fuel cell  Marine environment  Humidification  Sea water  Contamination

1 Introduction The need for a healthy environment is pushing researchers across the globe to look out for technologies that are clean and green. One such promising technology is the Polymer Electrolyte Membrane Fuel Cell (PEMFC) Technology that derives power from hydrogen. PEM fuel cells are widely considered as a substitute to the existing fossil fuel based power systems. The emissions from fossil fuel based power systems can be abated to a greater extent by substituting fossil energy with hydrogen energy [1]. To address this issue of emissions and reduce the burden on conventional power sources, PEM fuel cells are being considered as suitable power packs for both stationary [2– 4] and transport applications [5–8]. Many research groups are focusing towards contamination, prevention and mitigation strategies to increase the durability of the PEMFC under practical conditions. Apart from technical world, the understanding of PEM fuel cell as a promising energy source for marine applications also needs considerable understanding [9]. During the operation of a PEM fuel cell, the roles of fuel H2 and oxidant are pivotal in obtaining the desired power output. The equations corresponding to voltage that could be generated from the fuel and oxidant are given by the following equations. H2 ! 2Hþ þ 2e

Eo ¼ 0 V ðAt the anodeÞ

ð1Þ

1 O2 þ 2Hþ þ 2e ! H2 O Eo ¼ 1:229 V ðAt the CathodeÞ 2 ð2Þ

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1 H2 þ O2 ! H2 O Eo ¼ 1:229 V ðOverall ReactionÞ 2

ð3Þ

H2 is supplied to the anode region of fuel cell from compressed cylinders, metal hydride canisters, online reformers etc., On the cathode region of PEM fuel cells, for supply of oxygen, air compressors/blowers are used conventionally, due to their economic leverage over the use of compressed oxygen cylinders. Impurities on the anode fuel and cathode oxidant are expected to cause performance degradation due to several reasons. The fuel side impurities like CO, NH3, Sulphur-Carbon based, H2S, etc., arise mainly from the process used for producing the hydrogen and the raw materials [10–13]. To the best of our knowledge, most of the published work are related to the PEMFC performance due to the effect of contaminant type, its concentration in the fuel side. They have also reported that the contaminants drastically affects the catalyst layers and also the proton conductivity of the membrane. However, the impurities present in the oxidant feed also affect both electrode and membrane of PEMFC. The major contribution of impurities at the cathode side arises from NOx and SOx compounds, which are predominantly from industrial and automotive emissions in the atmosphere on land surfaces when compared to atmosphere on sea surfaces [9, 10]. There are also studies to mitigate the effect of these contaminants, but only on closed cathode fuel cell systems in normal atmospheric operations [11]. Blowers are conventionally used as sources for the supply of air to the fuel cell stacks of higher capacity. Apart from blowers, compressed cylinders can also be used to supply either oxygen or air. The impurities from fuel inlet can be contained by purifying the H2 to high pure quality (99.99 % purity) using methods like Pressure Swing Adsorption, Metal Hydride adsorption, Ag-Pd membranes, etc. [12, 13]. However, on the cathode side the impurities are difficult to purify and economically not feasible, especially when oxidant air in large quantities from atmosphere is supplied to the fuel cell stacks. The choice of power for marine applications is a challenge by itself and in addition the power sources for marine applications operate far away from the land regions, in the middle of seas and oceans, leading to difficulty for repair and maintenance. Hence they require uninterrupted power supply for a longer duration. Normally, hydrocarbon based fuels like diesel and Gasoline are used for on board marine vehicles. However the emission that include mainly NOx, and SOx, are harmful for the marine species [14]. Hence, PEM fuel cells find more prominence in usage for on board marine vehicles due to several advantages like less pollution, modular nature, easy start up, high efficiency, etc., However, the major advantage could be attributed to its uninterrupted power supply at zero emissions. Hydrogen is

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considered as clean and ideal fuel for PEM fuel cells that have been successfully deployed in submarines [14–16]. Similarly the comprehensive usage of fuel cells navy, merchant and passenger ships, has been carried out by several institutions and/or organizations in the recent past [17, 18]. Usually for submarine vehicles the requirements for hydrogen and oxygen are met by the use of cylinders, metal hydride canisters, compressed liquids and other oxidants [18]. However, given the weight and volume constraints for ships and submarines, it would be beneficial to try and reduce the pay load in terms of the number cylinders being carried for the supply of hydrogen and oxygen. Hydrogen can be alternatively generated from on board reformers or can be carried in liquid forms, however has limitations on production capacity with respect to the demand. On a marine vehicle, use of blowers for the supply of air to fuel cells can be an ideal choice while reducing the weight, volume and cost constraints and also maintaining the required oxidant flow with the help of controllers, to the fuel cell stacks. This has been observed as a good approach as long as the blower is operated in closed environments. But the fact that environment around marine vehicles is not as conducive as it would be on land due to the surface mists on sea, which are Cl-, Na?, SO42-, Mg2?, Ca2?,K?,HCO3-,Br-, etc. [19]. Further the presence of these ions on the anode or cathode side of a fuel cells would affect the performance of the fuel cell [20, 21]. The presence of these ions can be averted on the anode region as long as the fuel H2 is supplied from high pure closed containers. However these ions could be of an issue at the cathode side, when the environmental air from a blower is used for the fuel cell stack [22]. The common ions that are present in sea water either individually or together is detrimental to the performance on the fuel cell by poisoning the components like catalyst, membrane, electrode, etc. [26]. It has been observed that the presence of Cl- ions will reduce the active electrochemical area by the adsorption of them onto the Pt particles. In addition, an increase in the charge and mass transfer resistance has also been observed due to Clcontamination [23]. The reactions related to the impact of Chlorine in fuel stream on PEM fuel cells are given in Eqs. 4–7. Cl2 ðgÞ þ H2 O ! HClO þ HCl þ





ð4Þ

HClO þ H þ 2e ! Cl þ H2 O

ð5Þ

H2 ðgÞ  2e ! 2Hþ

ð6Þ

Pt

Cl2 þ H2 ðgÞ þ H2 O ! 2HCl þ H2 O

ð7Þ

The air contaminants that can affect fuel cell performance are NOx and SOx compounds. However, in the

J Mar Sci Technol

marine environment, their concentration in sea water mists is very marginal, compared to that on land regions [24, 25]. This concludes that the major ionic contribution to sea water mists is from NaCl. Although the presence of sodium chloride in the air stream of fuel cells affects the performance, the impact is negligible if the system is operated at low current densities. However, if the duration of the fuel cell operation, under these chloride contaminants in the cathode reactants is longer, then there is a possibility for considerable performance drop in the fuel cell [26]. To our knowledge not much work has been reported in terms of operating of PEM fuel cells under sea environment, and there exists some reports based on the Molten carbonate fuel cells (MCFC) [25]. As MCFCs operate at higher temperatures they are less prone for contaminants. However, high-temperature operation is also a cause to worry, especially for marine applications, where the safety, damage control features, and balance of plant systems could be weak when compared to that on land region operations. Thus in view of the operating temperatures and contaminants, low-temperature PEM fuel cells can be of greater advantage, given the fact that their operating temperatures never exceed beyond 80 °C. In the present study, a PEM fuel cell stack of 100 W capacity has been operated under simulated sea water environment. The sea water environment is created by taking a pool of sea water in a container and with mild air blowing over the surface of this sea water container, as shown in Fig. 1. The performance of the fuel cell stack has been evaluated and compared in domestic air environment and as well under sea water environment. The humidity available from the surrounding water in the container has been utilized by air feed that is supplied to the cathode region in fuel cell. A preliminary investigation and a

detailed analysis of the vagaries in fuel cell performance have been studied and are reported. The process for recovering the fuel cell stack after contamination to sea water environment has also been reported.

Fig. 1 PEM fuel cell stack

Fig. 2 PEM fuel cell stack under humid environment

2 Experimental study An indigenously developed fuel cell stack with open cathode structure is used in the studies. The stack consists of 24 numbers of 90 sq.cm of electrode area, developed by a proprietary process at CFCT laminated with Nafion 1135 membrane from M/S Dupont denomours. The anode electrodes are loaded with 0.25 mg/sq cm of Pt/C catalyst, while the cathode electrodes are loaded with 0.5 mg/sq cm Pt/C. The PEM fuel cell stack used for this study is as shown in Fig. 1. The air is supplied using a small fan, which caters to the need for both oxidant and as well as coolant, to take away the heat generated from the stack. The fuel is supplied from compressed hydrogen cylinder that has 99.999 % purity ensuring no contaminants to be present on the reactant supplied to anode region. The inlet hydrogen pressure is kept at 2 bars with a flow rate maintained between 0.5 and 2 lpm to the fuel cell stack. The stack is operated by increasing the load in steps using an electronic load box. The experimental setup for the fuel cell operation under various environmental conditions is given in Fig. 2. The experimental facility consists of a PEM fuel cell stack, fans, water container, a small blower, an IR lamp, electronic load, etc. Initial trial runs were conducted to get the baseline performance of the data using normal water. The sea environment experiments were conducted by replacing the water in the container to sea water and a fan is provided in order to facilitate the movement of sea water mists onto the fuel cell stack. The temperature of the water

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contained has been increased to create sea water vapors around the stack by using an IR lamp. The corresponding fuel cell stack voltage developed is monitored. Individual cell voltages are also noted down to evaluate the single cell performance. A reverse scan is performed by decreasing the load in steps for obtaining the polarization curve to see the reversibility of the cells during and after contamination, if any. An electric load box (chroma 60201) is used for the purpose of managing the load supplied to fuel cell stack. 2.1 Fuel cell activation The fuel cell stack consists of 24 cells of 90 sq.cm electrode area. Hydrogen from compressed cylinders, air from fan, a humidifying pot for hydrogen humidification, water tub for humidifying air supplied to stack, flow controller, 12 V DC power supply and an electronic load box are used for operating the stack. The experimental study involved activating the fuel cell stack under various conditions. The PEM fuel cell stack as shown in Fig. 1 is initially tested for its performance under dry reactants condition (reactant gases have been supplied as de-humidified). The schematic of that experimental setup is shown in Fig. 3. Further the stack was tested under humidified reactants condition, by bubbling the hydrogen through a humidification pot. In this case of air, humidification is obtained by using an external fan to blow the moist air (vapors) over the surface of water around the fuel cell stack under which it is operating, as given in Fig. 2. Finally, the water in the tub is changed to sea water when the sea water environment is desired. The schematic for water and sea water type humidification setups is shown in Fig. 4. The performance of the fuel cell stack is analysed under three different experimental conditions. Initially the fuel cell stack is supplied with dry reactants to obtain the dry run performance of the stack. Further for humidified reactant studies, H2 is humidified using a humidification

Fig. 3 Experimental setup (Dry Run)

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pot and air is humidified using moisture as obtained from water surrounding the fuel cell stack. The performance of the fuel cell stack under normal water ambience conditions is referred as the baseline performance for this study. It is observed that the performance of the stack came down when it is operated under simulated sea-air conditions. This may be attributed to the effect of ions present in the mists over sea water that can result in the degradation of polymer electrolyte membrane, which are also reported in the literature by Lope T et al. and Li H et al. [27, 28]. In order to remove the ions that accumulated inside fuel cell stack, which can contaminate the stack, water washing is adopted where pure demineralised water at a particular flow rate of 20 ml per min is pumped through the stack using a small water pump. Later the air flow channels are spray cleaned with water. The water washings are collected and evaporated to dryness, and the residue was tested for various ions.

3 Results and discussion The PEM fuel cell stack is initially operated under dry reactants condition to check the health of the fuel cell stack, and followed by operating it under humidified reactant conditions. However, in a fuel cell stack with open cathode structure, controlled humidification of the air is not possible and the humidification depends predominantly on the ambient conditions. Although there are several publications on performance of open cathode PEMFC, for the first time, we report here the results of the performance of a PEMFC stack with an open cathode structure, when operated under sea water environment. It is found that the performance of the PEMFC stack reduced on continuous operation and may be attributed to the deposition of salt on the electrodes. By washing the electrode with water, it is possible to recover the performance of the PEMFC stack. The 24-cell fuel cell stack used in this study gives an open circuit voltage of 21.5 V and delivering around 24 W powers at 4 A using dry reactants. The polarisation curve is shown in Fig. 5. Thus taking a lead from the fuel cell stack’s performance under dry conditions, the stack is then tested with humidified reactants and the enhancement in its performance, if any, under the humid conditions is studied. Further the fuel cell stack is operated under humid reactant conditions as given in Sect. 2. The fuel cell stack shows improvement in its performance under these humid conditions and the pertinent performance data are represented as polarization curve in Fig. 6. The performance enhancement is observed after 48 h of break in operation. However, the immediate performance of the stack right after dry conditions under humid reactants is around 36 W

J Mar Sci Technol Fig. 4 Schematic of the tub based humidification for the fuel cell stack (water/sea water conditions)

Fig. 5 Dry run polarization plot

(6 A at 6 V stack voltage). This indicates an improvement of over 50 % in the power drawn from its last dry operation performance of 24 W (4 A at 6 V stack voltage). Further it can be observed that the final performance of stack after 48 h of break in activation is around 60 W (10 A at 6 V stack voltage). This indicates almost an improvement of 150 % in power delivered by fuel cell stack over its initial dry conditions operation. Thus this performance will be considered as the baseline reference performance of the fuel cell stack in this study. The behaviour of fuel cell stack under sea water environment is studied by replacing the plain water with sea water in the tub as shown in Fig. 4. The corresponding performance data in the form of polarization curve is given in Fig. 7.

Fig. 6 PEM fuel cell performance enhancements under humidified conditions

It can be observed that the performance of fuel cell stack under sea water environment decreases from 56 W (7 A at 8 V stack voltage) to 24 W (3 A at 8 V stack voltage). The stack is operated under sea environment for about 48 h under similar conditions as it is done with plain water humidification. The deterioration in the stack’s performance has been more than 50 %. The drop in performance can be attributed to the sea water environment under which the stack has been operated. It may be assumed that the presence of several ions that are present in sea water mists [27, 28], could be carried from the sea bed to the air feed to the stack, resulting in performance degradation. A water wash on the cathode portion of fuel cell stack is done to help collect the particulate residues settled inside the fuel

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Fig. 7 Performance of PEM fuel cell stack on sea water environment

Fig. 8 EDAX elemental analysis of residue collected from PEM fuel cell stack wash

cell stack. This particulate matter is tested for the various elements present inside fuel cells stack under sea water operation. The cathode is washed with water using a water spray gun. The washed water is collected, evaporated to dryness and the reside is analysed using EDAX which is shown in Fig. 8. The ions detected in EDAX [Sulphur (S), chlorine (Cl), Sodium (Na), Calcium (Ca), Potassium (K)] indicate a composition similar to that of sea water mist [16]. Especially the presence of Na and Cl prove the assumption that salt has been deposited onto the cathode region of fuel cell and the performance degradation is due to presence of ions from sea water on the cathode. As discussed in the earlier section, water washing has been carried out by spraying water gently on the cathode openings of the fuel cell stack. After thorough washing, the fuel cell stack is tested for its performance under humid conditions using plain water environment. The graph

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Fig. 9 Performance of PEM fuel cell stack after water wash on the cathode side

Fig. 10 Comparison of overall power delivered by stack under various environments

indicating the performance enhancement/recovery as polarization data is given in Fig. 9. The recovery of the stack after water wash in terms of power output is given in Fig. 10, which shows that, 56 W of power can be obtained after the treatment, which is almost 150 % improvement in power delivered. Table 1 represents the summary of experimental studies conducted with reference to the load applied and the maximum achievable performance by the stack under various conditions. It can also be assumed that the various ions present in the sea water could have occupied the electrode surface hindering the proton conductivity inside the cell. This loss in proton conductivity might have resulted in the performance drop. Further it can be viewed that the particulate matter has no detrimental effect on the catalyst sites in fuel cell electrodes, or cannot penetrate to the level of catalyst, which are usually vulnerable to NaCl poisoning.

J Mar Sci Technol Table 1 Overall experimental studies summary Experimental condition

Reactants supplied state

Maximum load applied

Maximum achievable performance

Dry reactants

H2: dry

6A

24 W

Air: dry Humidified reactants (normal water ambience)

H2: humidified using humidification pot Air: humidified using the moisture surrounding the stack

10 A

60 W

Humidified reactants (sea-water ambience)

H2: humidified using humidification pot

3A

24 W

7A

56 W

Air: humidified using the moisture surrounding the stack Humidified reactants (after cathode water wash-normal water ambience)

H2: humidified using humidification pot Air: humidified using the moisture surrounding the stack

P3 ¼ 11:5

Fig. 11 Power rates for the PEM fuel cell stack under variable environments

The power built up during recovery and drop during contamination, in the present fuel cell under various environmental conditions is given in Fig. 11. As described earlier the initial performance output of the fuel cell stack under humid conditions is around 60 W, as given in Fig. 10. This performance drops to as low as around 22 W when operated under sea water environment. Upon water washing of the fuel cell stack, the fuel cell stack recovers up to delivering a performance of around 55 W under normal water environment. The rates at which the power developed and dropped have been calculated by linear approximation of the power graphs as shown in Fig. 11, and the corresponding equations are given below (Eqs. 8–10). Initial power built up under water environment (P1), Sea water environment (P2), and after water wash with water environment (P3) are given below. t P1 ¼ 7:4 þ 20:47 ð8Þ 10 t P2 ¼ 5:7 þ 56:93 ð9Þ 10

t þ 2:5 10

ð10Þ

where P is the power delivered and t is the operation duration in hours. The slope of these linear equations will define the rate at which the power varied. It can be observed from Eq. 8 that the initial power built up rate P1 is around 7.4 W per 10 h. This rate then dropped to around -5.7 W per 10 h, indicating a performance drop, when fuel cell is operated under sea water environment delivering power P2. However, when water washed thoroughly on cathode region, the power P3 built up rate picked up to 11.5 W per 10 h. An increase in rate by 4.1 W per 10 h is observed from the initial rate P1. This increase in overall rate of power built observed from P1 to P3 could be due to the wash off of not only the impurities as carried from sea water mists but also those present from earlier operations.

4 Conclusions In principle, operating a PEMFC stack in sea water environment should be advantageous as the humidity is expected to help in its operation. However, this study clearly demonstrates that the performance of the PEMFC with open cathode structure can be used in such applications as the salt in the sea mist can deposit on the surface of the cathode and reduce its performance. The study also demonstrates that the performance can be recovered on washing the cathodes of the fuel cell stack with water indicating that the ions of the salt have not poisoned the catalyst. It is clear from these studies that the fuel cell stack design is important and enough and precaution needs to be taken to prevent ingression of salt mist in the cathode air stream like use of fine filters, sieves and etc., across the air inlet stream. Further it is required to assess the effect of practical implementation of the salt/mist filtering techniques, across air stream, on the fuel cell performance.

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J Mar Sci Technol Acknowledgments The authors would like to acknowledge Dr. G. Sundararajan, Director ARCI for his encouragement. This work was carried out under a project which was supported under the DST– RCUK initiative.

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